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•Two of TV's more popular contests compete tonight, as the three-hour finale of The Biggest Loser (NBC, 8 ET/PT) goes up against the final-three performers on American Idol(Fox, 8 ET/ PT). If you decide to watch Loser and record Idol, remember to add a few extra recording minutes, because when it comes to ending their show on time — a fairly basic TV skill — the Idol producers are either incapable or indifferent.

•It's prom night on 90210 (CW, 9 ET/PT), and the school is using a Hollywood theme. Makes you wonder whether somewhere in the Midwest, a school is using a 90210 theme.

•Leonard Nimoy follows his Spock stint in J.J. Abrams' Star Trekwith a guest debut on the season finale of Abrams' Fringe (Fox, 9 ET/PT). He plays the much-discussed, but until now unseen, William Bell, Walter's former lab partner and the man who may be behind a season's worth of fringe science freakiness. He's joining a show that is much improved since its premiere, thanks to story streamlining and careful nurturing of the main characters. Of course, when last we saw them, one of those characters, John Noble's Walter Bishop, was wandering away with a Watcher. But you have to assume he'll be back, because without Walter, there's much less reason to watch.